The invention is directed to a method for performing verification measurements and corrections of magnetic fields in magnets for Nuclear Magnetic Resonance imaging machines, comprising the following steps:
making a test member having such characteristics as to generate predetermined and known Nuclear Magnetic Resonance images;
obtaining a Magnetic Resonance image from said test member;
comparing the known theoretical image with the detected image and/or the received signals wherefrom the detected image is reconstructed with the corresponding set of signals, related to the known theoretical image in any of their processing or imaging steps;
determining the differences between the detected image and the known image or between the set of detected signals and the set of signals corresponding to the known image;
determining correction parameters, i.e. the correcting magnetic charge and/or the number of correcting magnetic charges having a predetermined value, or the volumes of ferromagnetic material and their position on the magnetic structure.
Nuclear Magnetic Resonance imaging machines use electromagnetic echoes of previously excited nuclei to retrieve therefrom information for imaging. In order to obtain echo signals allowing to reconstruct images sufficiently corresponding to reality, at least a considerable number of the nuclei of the material being examined must be oriented with substantially parallel spins. To this end, static magnetic fields are used which must be comparatively intense and constant within a predetermined volume corresponding to a body part to be examined.
At the same time, additional magnetic fields are applied, the so-called gradients, which are used to select certain sections of the body part to be examined and to create a parameter for identifying the signal received from individual portions of the section under examination so that signals can be ordered for two- or three-dimensional imaging.
The tolerances required for a good correspondence between reality and the reconstructed image are very small, being of the order of a few millionths deviations from nominal values. Moreover, since the magnetic fields in use are relatively intense, the constructions of magnets have a considerable size, whereby the physically huge structure hinders the required construction accuracy. This is particularly relevant for the so-called permanent magnets, in which the magnetic field is generated by permanently magnetized materials and not by induction generators.
After manufacture, magnets are submitted to fine calibration of the magnetic field, the so-called shimming, which is designed to correct construction inaccuracies of the individual parts of the magnet, and includes measuring the static field by a probe, detecting aberrations with respect to nominal values and disposing magnetized or ferromagnetic correcting elements in several appropriate areas of the magnet structure as determined from the differences between the nominal values and the actual values of the field within the volume designed to accommodate the body or the body part under examination. Shimming is performed during fabrication at the manufacturer""s site. This technique is described in greater detail, for instance, in patent application SV98A000015 filed by the applicant hereof.
Nevertheless, frequently and unavoidably, upon installation at the customer""s site, manufacture settings appear to be changed or perturbed. This is especially caused by stresses acting on the big metal masses which form the magnetic structure, especially as this structure has a modular construction, or made of several parts fastened together.
Further, the personnel using the machines is not sufficiently qualified to verify their operating accurateness during use, except in a few exceptional cases. Hence, little aberrations might remain unnoticed.
Due to this, a problem arises to make functional testing of machines easier, at least below a certain accuracy degree, in an easy and safe manner, even for personnel less qualified than the personnel charged of fabricating and testing the machines at the manufacturer""s site.
Document EP-A-0230027 discloses a method and a device according the above described method and which attempts to solve the above disclosed problems. Also U.S. Pat. No. 5,545,995 and U.S. Pat. No. 5,055,791, disclose method of the kind described above.
The known disclosed methods, suggest solutions to the problem of testing and correcting imaging aberrations and anomalies in which the image of the phantom is determined by a combination of several coefficients of the theoretical mathematical function which describes the field and which combination of coefficients includes coefficients of low and high order. Carrying out the correction of the field under these circumstances requests very long calculations which are not necessary to achieve a practical level of precision of the field. Indeed in the disclosed solutions the structure of the phantom is not suited to the structure of the function describing the field being normally a spherical polynomial expansion. The images of the phantom and the aberrations thereof from the theoretical image are not determined only by a small number of low order coefficients of the polynomial expansion. Thus as stated above the mathematical comparison of the real image of the phantom and of the theoretical image and the calculation of the correction to be applied to the field are very complicated and time consuming.
The invention has the main object to allow fast functional testing of the magnetic structure of Nuclear Magnetic Resonance machines, particularly after installation, in a relatively fast and simple manner.
The invention has the further object to allow an at least partly automatic correction of aberrations and anomalies occurring after installation without requiring special laboratory equipment, which is complex and costly and may be only used by very highly knowledgeable and specialized personnel.
The invention also aims at setting the bases for implementation of several automation degrees of the correction to be performed, with the smallest number of operation steps which might be carried out by an average specialized technician, specifically trained in the maintenance of one or more specific machines.
The invention achieves the above purposes with a method for performing verification measurements and corrections of magnetic fields in magnets for Nuclear Magnetic Resonance imaging machines, in which
A mathematical theoretical function describing the field is choosen, particularly a polynomial expansion, preferably a spherical polynomial expansion,
A phantom is provided with a structure suited to the said function describing the field and which produces one or more selected images, each of these images being correlated to one or to a limited number of selected low order coefficients of the function describing the field, such that the differences between a theoretical image of the phantom and the real image of the phantom depends only to the one or to the limited number of selected coefficients of the function describing the magnetic field.
Thus by selecting a specific mathematical structure for describing the field, for instance and preferably but without limitation, a representation of the field by spherical harmonics, the above method may be refined so that several orders and degrees of harmonics, and hence of field coefficients may be examined individually.
In this case, the method provides that the test member has elements which do not emit Nuclear Magnetic Resonance signals and are defined transparent in the following text and claims , which are related to a definite harmonic or to precise coefficients of a certain order of a mathematical description of the field by a polynomial expansion, preferably by spherical harmonics, there being provided means for application of so-called reading gradients of the magnetic field, which only detect the echo signal along certain directions, said directions being selected in such a manner as to suppress the contributions from the magnetic field described by coefficients other than the ones being examined.
Preferably but without limitation, the describing polynomial expansion is selected in spherical harmonics. Nevertheless, other polynomial expansions may be more or less suitable for the purpose, also depending on the symmetries of the field and hence of the magnetic structure.
In accordance with a preferred embodiment of the method, the latter includes the following steps:
a) detecting the image of the predetermined test element;
b) symmetrizing deviations between the actually detected image and the nominal, ideal one, with respect to the center of the image and/or to the predetermined origin of the coordinate system for the mathematical description of the magnetic field;
c) defining a curve of symmetrized deviations having the relevant field coefficients as a variable;
d) determining a polynomial for approximating the curve of symmetrized deviations, deriving from the differences betweem the actual image and the ideal, theoretical image;
e) determining the coefficients based on the system of the two consistent equations of steps c) and d);
f) computing, from the mathematical description in spherical harmonics of the field, such number, magnitude and position on the magnetic structure of the correcting elements as magnetic charges or volumes of ferromagnetic material, as to bring the measured values of the field coefficients back within the nominal values corresponding to the homogeneity characteristics of the field required for detecting useful Nuclear Magnetic Resonance images;
g) manually positioning such charges on the magnetic structure.
Possibly, once correction has been performed, steps a) to f) may be repeated to verify the effectiveness of said correction and, when necessary, an additional correction step g) may be performed.
The elements for verifying the different coefficients may be progressively mounted on the test member or fixedly provided thereon.
Each of these elements corresponds to at least one harmonic or to at least one set of coefficients and are made consistently with the selected mathematical description of the field and/or eventually to the symmetries of the magnetic structure.
According to a first embodiment, the method provides a manual graphic comparison between the detected images and the theoretical images wherefrom the mathematical computation is performed to determine correction data.
In this case, the machine will have a theoretical comparison image, for instance stored therein.
Alternatively, the method provides that the machine determines quantitatively the differences between the ideal image and the detected image by computation of the distance in pixels of the point or portion or area of the detected image from the position of the corresponding point or area of the ideal image. In this case, the difference may be easily changed to an aberration or deviation quantitative value with respect to the theoretical image and directly used in the machine thanks to the processing hardware contained therein and to software for effecting comparisons, tests and error diagnostics. Hence, computation software, also stored in the processing hardware of the machine may compute the required corrections and indicate correction absolute data.
In these conditions, the operator would only have to physically position the correction charges, based on the magnitude suggested by the machine, in the position/s automatically determined by the machine.
In a specific embodiment, the method provides the use of a test element whose non-detectable portion has the form of a rib, stem or baffle whose longitudinal orientation is parallel to the static magnetic field, i.e. disposed in the axial section plane of the magnetic field when the latter is described with spherical harmonics. In a description by a polynomial expansion with spherical harmonics, this stem is related to (2 0) and (4 0) cosine coefficients.
From two crossed baffles which form an angle of 109.472xc2x0, symmetrically with respect to one of the two axes which describe the axial section plane of the magnetic field, the coefficients of the (2 2) cosine and of the (2 1) sine are obtained.
Similarly, from two crossed baffles which form an angle of 90xc2x0, symmetrically with respect to an axis which describes the coronal section plane of the static magnetic field the coefficients of the (2 2) sine are obtained, whereas from the same baffle configuration, with baffles disposed in the sagittal section plane of the static magnetic field, the coefficients of the (2 1) cosine are obtained.
When these coefficients are verified and, if needed, corrected, they are sufficient to ensure a good accuracy of the field to obtain the tolerances required for detecting useful Nuclear Magnetic Resonance images.
It has to be noted that (x y) typically defines the indexes of the polynomial expansion.
The invention is also directed to a test element including, separately or in combination, the above baffles.
In the preferred embodiment, such test element includes a central rectilinear baffle which is crossed, with reference to the plane parallel to a peripheral edge, by two crossed baffles, passing through the median area of the rectilinear baffle and forming, symmetrically with respect to the rectilinear baffle, an angle of 109.472xc2x0, whereas two additional crossed baffles are provided in the plane perpendicular to the rectilinear baffle and containing the other sides thereof, which form, symmetrically with respect to said rectilinear baffle, an angle of 90xc2x0 and intersect the rectilinear baffle in the central area.
Advantageously, the baffles consist of walls and are made of plastic, particularly of the so-called Plexiglas.
In accordance with the illustrated preferred embodiment, the different baffles made of a material which emits no Nuclear Magnetic Resonance signal are mounted in a fixed manner inside a plastic container filled with a liquid which emits a Nuclear Magnetic Resonance signal, e.g. water or other liquids or solutions.
It has to be noted how the creation of contact surfaces between materials differing with respect to Nuclear Magnetic Resonance signals is relevant to generate image contrasting imaging areas, having predetermined shapes. Obviously, said preferred construction allows the method to be implemented even when the materials composing the baffles and the filling are inverted.
The box or container have a filler opening with sealing means.
The advantages of the present invention are apparent from the above disclosure. Thanks to the verification and correction method, the homogeneity accuracy of the magnetic field may be periodically controlled in an easy manner, manually and/or with different automation degrees. This feature is primarily important for obtaining Nuclear Magnetic Resonance images with a good correspondence with reality, hence it is the basis to obtain diagnostically usable images.
Verification and, when needed, correction activities, may be automated to such an extent that they may be performed by an average specialized operator, without requiring any intervention of highly specialized personnel, which involve high costs both for the manufacturer and for the customer.
Further, verification and correction times are drastically reduced, thereby limiting downtime, which causes discomfort for users.
The required process and devices are relatively inexpensive, mainly involving the addition of software procedures in existing machine elements, which only need to be enhanced, and the provision of a set of correction permanent magnets, as well as of a phantom. The latter is the only construction element which has to be specially fabricated for an easy application of the method.
However, the phantom itself is in no way necessary, the verification and correction procedures being performable, by following the above method steps, with hand-made elements or improvised members.
Further improvements of the invention will form the subject of the subclaims.